Since the passage of the Clean Water Act in 1972 the federal government through the Environmental Protection Agency (EPA) has mandated progressively tighter controls over the quantities of pollutants and contaminants that are allowed to be released into our nation's waters. These progressively tighter mandates also encompass control of peak flows and/or volumes and the rate at which they can be discharged into existing water ways or drainage infrastructures. These resulting mandates and controls have given birth to new programs and procedures collectively referred to as storm water management.
Devices, systems and procedures that remove or reduce the pollutants and contaminates and/or control peak flows and volumes are often referred to as best management practices (BMPs). The BMPs can utilize natural, artificial or man-made techniques, and even combinations of either and/or both. Some examples of these BMPs include trash filters, sedimentation basins, retention and detention ponds, wetlands, infiltration trenches, grass swales, various types of media filters, and various types of natural filter systems including sand filters, and aggregate filters including natural and artificial wetlands.
BMPs typically use one or more mechanisms to remove the pollutants and contaminates. These mechanisms include sedimentation, filtration, absorption, adsorption, flocculation, stripping, leaching, bioremediation, and chemical processes including oxidation reduction, ion exchange, and precipitation.
Furthermore, storm water treatment systems can also be classified in relationship to the treatment level in which they are being used. In this respect, the term treatment is generally used to describe the unit processes that are used to reduce the quantities of pollutants and containments in storm water runoff.
For example, basic or pre-treatment typically refers to the removal of gross solids, sediments and larger debris through the processes of settling and screening. Enhanced or advanced treatment typically refers to processes or reducing targeted pollutants.
Filtration is a common form of enhanced treatment for storm water. Filtration utilizes a combination of physical, chemical, and biological processes. Types of filtration greatly vary dependent on the media use. Medias can be both inert and/or sorbent and are also strongly linked to natural biological processes that thrive in and/or around the media environment. Advanced filtration techniques especially include chemical and biological processes and generally include, but are not limited to processes that bring storm water in contact with plants including both macrophytes and microphytes. And the plants are both visible and invisible to the naked eye.
The reduction of nutrients that are conveyed via storm water runoff are in the forefront of the various pollutants of concern. For example; the EPA has mandated that the state of Florida reduce the overall pollutant discharge of the state by approximately ⅓. To this end the EPA has established a numeric nutrient criteria for all water bodies of the state for total phosphorus (TP) and total nitrogen (TN). These nutrient numeric values cannot be exceeded within these water bodies of the state. The result of exceeding these numeric values will result with the state of Florida being fined by the EPA.
Common pollutants typically found in storm water flow can include sediment, foliage, litter, nutrients, metals, hydrocarbons, and bacteria. Because pollutants vary significantly in their respective physical characteristics, a variety of techniques are applied to better address difficulties with capturing the pollutants. For example; capturing hydrocarbons, which are typically in a liquid state, require a different technique than capturing litter which are in a solid state.
Storm water treatment systems that make use of filtration media can vary significantly just by making use of different types of filtration media. Understanding that effectiveness will vary for different types of filtration media, has filtration media functions by combining the processes of physical filtration, sorbent or reactive surfaces, and biological activity.
Physical filtration will physically prevent the conveyance of solids, sorbent or reactive surfaces will chemically bond to the pollutants, and the biological activity will consume pollutants. Physical filtration whether by use of a filtration media or a screen can be an effective technique for capturing solids. Contained within and attached to the solids are a wide spectrum of pollutants of concern that can range from nutrients, to litter, to bacteria. A difficulty with physical filtration is that the collected solids can become so numerous that the rate of filtration is slowed to an ineffective flow rate. Servicing of the filtration media is typically required, and the servicing often requires the media to be replaced as a part of servicing.
Filtration media will have sorbent or reactive surfaces that vary in effectiveness depending on the type of media. For storm water treatment applications, the media is granular in shape. However, the reactivity of the media surfaces is limited by the number of receptors on the surface of the media.
Making use of media that has a lot of surface area will increase the number of available receptors. A common method to increasing the surface area of the media is using media that is small in granular size. However, the smaller the granular size of the media the more likely the media will be to bind due to the collection of solids within the media. In addition, the receptors can be binded by solids which prevent the water flow from reaching the surface of the media granules.
The biological activity of a filtration media is found within what is referred to the biofilm on the media. Micro-organisms will colonize the surfaces of the media and consume certain pollutants of concern. For example; a very effective technique for nutrient capture is to rely on the biofilm to consume the nutrient pollutant load. However, a difficulty with having a biofilm consume the nutrients is that the biofilm will require adequate time to consume the nutrients. A longer time frame in which the water is in contact with the biofilm will yield greater nutrient uptake. A media treatment system that can affect greater retention time within the media will achieve greater removal efficiencies.
Thus, the need exists for solutions to the above problems with the prior art.